1. Field of the Invention
The present invention relates to a gear shift control unit for an automatic transmission with which a temporary manual shift mode can be selected while driving in an automatic shift mode.
2. Description of the Related Art
There has been known an automatic transmission provided with a continuously variable transmission (CVT), a multistage transmission such as a planetary pinion mechanism, or the like in which a transmission gear ratio (transmission gear stage) is automatically set according to a driving state by setting a select lever in a D (drive) range and a manual shift mode where a driver can select any transmission gear stage by selecting the select lever in an M (manual shift) range, as is the case with a manual transmission (MT) vehicle.
There has also been known an automatic transmission provided with a temporary manual shift mode in which when a driver operates a down-shift switch or an up-shift switch provided separately in addition to the select lever during traveling in the automatic shift mode, the transmission gear stage is temporarily shifted down or up.
For example, Japanese Unexamined Patent Application Publication No. 2006-97789 discloses a technology in which when a down-shift switch or an up-shift switch provided in a front portion of a back face of a steering wheel is operated during driving in the automatic shift mode where the select lever is set in the D range, a gear shift mode is temporarily switched to the manual shift mode, and also the transmission gear stage is shifted down or up.
Furthermore, Japanese Unexamined Patent Application Publication No. 2006-97789 discloses a technology in which when a predetermined cancellation condition is satisfied, the gear shift mode is automatically returned from the temporary manual shift mode to the automatic shift mode.
In general, accelerated driving by depressing an accelerator pedal is set as a cancellation condition of the temporary manual shift mode as disclosed in the above-mentioned Japanese Unexamined Patent Application Publication No. 2006-97789. When the temporary manual shift mode is automatically canceled by accelerated driving to be returned to the automatic shift mode, accelerated driving is detected and the transmission gear stage is once shifted down. However, if the accelerator pedal is returned to change to constant speed driving, the transmission gear stage is shifted up. Therefore, even if the accelerator pedal is released after accelerated driving to perform decelerated driving, the gear shift mode has been switched to the automatic shift mode, and thus sufficient engine braking the driver expects cannot be obtained.
In the case of traveling on a curved road, for example, when a vehicle enters the curved road, down-shifting is performed and an engine brake is activated for deceleration, and, when the vehicle leaves the curved road, the accelerator pedal is depressed for acceleration. When the vehicle enters the curved road, load affected on a vehicle body is moved from a rear wheel side to a front wheel side through deceleration, which enhances cornering power of the front wheels, and the load affected on the rear wheels escapes, which decreases cornering power of the rear wheels, whereby maneuverability is improved. On the other hand, when the vehicle leaves the curved road through acceleration, the load affected on the vehicle body is moved to the rear wheel side, whereby the vehicle can run stably.
When down-shifting is performed in the temporary manual shift mode to enter a curved road, and then acceleration is performed when the vehicle leaves the curved road, the gear shift mode is automatically switched to the automatic shift mode. Therefore, when the vehicle enters a next curved road, the down-shift switch needs to be turned on again. Thus, troublesome driving operations are required.
Suppose that a vehicle travels on a road where curves continuously appear. If a next curved section cannot be predicted while the vehicle is traveling on a first curved section, and a curvature of the next curved section is larger than that of the first curved section (sharp curve), the driver leaves the first curved section through acceleration, and then try to turn the down-shift switch ON again and activate the engine brake upon entering the next curved section. Since the driver performs steering and braking operations at the same time, the down-shift switch cannot be operated, and thus the driver strongly depresses a foot brake so that the vehicle enters the next curved section in a deceleration state. Since the gear shift mode has been switched to the automatic shift mode, sufficient engine braking cannot be obtained, and excellent driving stability described above cannot be obtained.
Furthermore, even when the accelerator pedal is depressed to accelerate for leaving the curved section, it is determined that accelerated driving is performed since the transmission gear stage has been shifted up, and thus a response delay is generated until down-shifting is performed. During the delay period, the driver may have a feeling of torque insufficiency.